Implants to be inserted in the human organism must or should fulfil several requirements, e.g. that the implant should not release dangerous substances or cause allergic reactions from the patient. Obviously, this applies in particular to the implants intended for permanent or lasting placement in the organism, but neither pin-fixations which are often removed after a few weeks or months should release dangerous substances or cause allergic reactions. Pin-fixations could be pins or nails which are inserted through the tissue to a bone, and these pins or nails are mutually connected to an outer support, the ends of e.g. a broken bone being secured in a correct position and the load being transmitted by the pins and support. Pin-fixations may also be used alone in a bone in order to hold together bone parts when healing and in this case, the pin-fixations will often be left in the body, thus avoiding further operations which may be traumatic and furthermore create more cicatricial tissue.
However, many implants have been produced from materials which do not fulfil this requirement, for example, one material often used is an alloy of cobalt, chrome and molybdenum (CoCrMo) (e.g. Vitallium® containing 60% Co, 35% Cr and 5% Mo). It has appeared that the cobalt content, among others, in CoCrMo is dissolved and diffused out of the implant and into the bloodstreams, which is strongly undesirable since it may result in poisoning and injuries of organs, including the heart. Another material often used for implants is stainless steel alloyed with nickel which is also an undesired substance as it may cause allergy. On the other hand, pure titanium is relatively tissue-friendly and can be used for implants essentially without inconvenience in respect of allergy or poisoning. However, pure titanium is less suitable for implants which are exposed to major mechanical forces, such as hip implants, as implants of pure titanium do not have the same good strength properties as the above-mentioned materials and thus, implants of pure titanium are to a higher degree exposed to fracture than implants of the other materials. Therefore, different titanium alloys have been developed with improved strength properties, but also in this case, the alloy substances may cause allergy or poisoning.
In prior art, several attempts have been made to prevent the implant from releasing poisonous substances or causing allergic reactions. It is thus well described how the implant can be coated with a tissue-friendly material. WO 99/65537 discloses a metallic implant with a surface or a surface coating consisting of several layers of e.g. tantalum in layer thickness of 5 μm or more. This implant has the substantial disadvantage that the coating may peel off as so far it has in practice been impossible to make the layers adhere properly to each other and to the subjacent base body. Of course, the peeling is particularly a problem with implants which are driven into a bone with great force or which are exposed to major loads when inserted in the body. In addition to lacking the desired function as protection against release of unwanted substances from the implant, the peeling of the coating has further the disadvantage that the fixation of the implant is defective in these areas.
In order to avoid peeling of a coating, U.S. Pat. No. 4,743,308 discloses a process of passivation of a metal alloy, especially a Co—Cr—Mo alloy (Vitallium®) with a coating of tissue-friendly material, such as a noble metal, and exposing the coated surface to a bombardment by an ion beam which drives the coating into the metal alloy such that there is no surface layer which can peel off. However, the ion beam bombardment causes that the surface of the implant is not impervious, but almost porous, as it will be spotted with small so-called pin-holes from the ion beam bombardment, and thus, there is a risk that the implant still releases noxious substances.
Furthermore, certain types of implants are exposed to very heavy mechanical loads, e.g. implants for femurs, hip socket and knee, and these implants must therefore fulfil some severe strength conditions. This is becoming more important as in addition to elderly people needing implants because of attrition and fracture, e.g. if they suffer from osteoporosis, there is an increase in the number of young people who need such implants due to acute injuries and attrition resulting from extreme sport activities or the like. It has turned out that knee and hip implants have a durability of 10–15 years which is sufficient in many cases for elderly people, but not for young people. This is due to the fact that it is difficult and often impossible to re-operate, so often an implant cannot be replaced by another as the operation is a major surgical intervention which may be disabling when repeated.
U.S. Pat. No. 5,415,704 discloses a surface hardened, metallic implant where the implant is produced from a metallic alloy added with a dissolved, slightly oxidizable or nitridable metal, such as tantalum. The object is to form an oxidized or nitrided surface layer which can both seal the surface in order to prevent release of poisonous substances and harden the surface in order to obtain a high abrasion resistance of the implant. However, this implant does not have a diffusion-tight, uniform surface and the hardened surface might crack at a bending or fluctuating load as the surface will be relative brittle and cannot follow the movements of the base body.
WO 99/26673 discloses an implant provided with a surface layer. The thickness of the surface layer is chosen such that it is less than the critical defect size for the actual material and stresses. The surface layer may consist of calcium phosphate or an oxide of titanium, zirconium or tantalum. It is indicated that the thickness of the surface layer preferably is less than 5 μm. The object of this invention is to provide a maximal thickness of a surface layer of a material with low strength to prevent the formation of fissures in the layer which may initiate cracks in the base body such that the strength of the implant will be less.
Finally, DE 19940970 discloses a process for an implant of titanium or a titanium alloy with a protection layer of TiO2 and further a surface coating with calcium. However, this patent was published only after the priority date of the present application.